Carey Mulligan is a terrific actress. The luminous Londoner has earned raves for movies such as “An Education” and “Never Let Me Go,” and she could easily have spent her summer shooting some blockbuster or other. Instead, she’s in a small off-Broadway theater, playing a schizophrenic woman in “Through a Glass Darkly.”

Mulligan is perfectly fine, of course, but even she can’t save the show.

The title will be familiar to cinephiles: Jenny Worton’s new play is based on the Ingmar Bergman flick that won the Oscar for Best Foreign Film in 1961. He’s lucky to be dead and safe from this fiasco.

As an adaptation, “Through a Glass Darkly” is clueless about Bergman’s aesthetics and themes. But we didn’t trade up: On its own terms, it’s a ham-fisted melodrama. Talk about lose-lose.

Just released from the hospital, Mulligan’s Karin is vacationing on an island with her family, including loving husband Martin (Jason Butler Harner) and distant father David (Chris Sarandon). Quickly, though, Karin has a relapse, hearing voices that announce the imminent arrival of God.

As those who saw the 2008 Broadway production of “The Seagull” know, Mulligan has incisive stage smarts. She has the uncanny ability to be simultaneously brooding and radiant, and here she shades Karin’s descent into madness with an almost painful sympathy.

It’s telling that the name of Karin’s younger brother (Ben Rosenfield) has been changed from Minus to Max — but more is less.

Worton and director David Leveaux (who helmed the current revival of “Arcadia”) needlessly amplify Karin’s inner voices so we, too, hear the din filling her head. The father’s metaphysical dread is gone, and the siblings’ ambiguous relationship is now graphically spelled out.

It’s as if the audience was assumed to be too dumb to handle gray areas.

But this is nothing compared to the ending. If you haven’t seen the movie, it’s merely half-baked. If you have, it’s confounding. What a colossal missed opportunity.